Hydrogen absorbing alloy electrodes serving as the negative electrodes of nickel-hydrogen batteries are prepared by pulverizing a hydrogen absorbing alloy ingot to obtain a hydrogen absorbing alloy powder, admixing a binder with the powder and shaping the mixture in the form of the electrode. The hydrogen absorbing alloys heretofore developed include those of AB.sub.5 -type rare earths having a crystal structure of the CaCu.sub.5 type, such Mm--Ni alloys, and TiNi.sub.2 alloys having a Laves-phase structure of the C14-type or C15-type.
With the nickel-hydrogen batteries having a hydrogen absorbing alloy electrode as the negative electrode, a gas-phase reaction and an electrochemical reaction proceed at the same time on the surface of the hydrogen absorbing alloy owing to the contact of the alloy surface with an alkaline electrolyte. More specifically, in the relationship between the hydrogen pressure and the temperature, hydrogen is absorbed by the alloy, or the alloy desorbs hydrogen (gas-phase reaction). In the voltage-current relationship, on the other hand, application of voltage (charging) permits the alloy to absorb the hydrogen produced by the electrolysis of water, and the delivery of current (discharging) oxidizes hydrogen to form water (electrochemical reaction). The properties of the alloy surface are therefore important in improving the performance of the nickel-hydrogen battery.
Accordingly, to improve the activity of the hydrogen absorbing alloy for use in nickel-hydrogen batteries, it is conventional practice to immerse a hydrogen alloy powder in an aqueous acid solution for surface treatment as disclosed in JP-B-225975/1993, or in an aqueous alkaline solution for surface treatment as disclosed in JP-B-175339/1988. The surface treatment removes an oxide film formed in the surface layer portions of the alloy particles, permitting rare-earth elements (such as La) to dissolve out and forming a nickel- or cobalt-rich layer in the surfaces layer portions of the particles, whereby the alloy is given improved electrochemical catalytic activity.
However, we have found that the conventional surface treatment still fails to afford sufficient activity although forming the nickel- or cobalt-rich layer in the surface layer portions of the alloy particles.
An object of the present invention is to provide a hydrogen absorbing alloy powder having higher activity than conventionally, a process for producing the powder, a hydrogen absorbing alloy electrode wherein the power is used, and a metallic oxide-hydrogen battery comprising the electrode.